ttx-diff

March 11, 2026 ยท View on GitHub

PyPI Python Version MIT/Apache 2.0

A tool for comparing font compiler outputs between fontc (Rust) and fontmake (Python).

Overview

ttx-diff is a helper utility that compares binary font outputs from two different font compilers:

  • fontc: The Rust-based font compiler from Google Fonts
  • fontmake: The Python-based font compiler

The tool converts each binary font to TTX (XML) format, normalizes expected differences, and provides a detailed comparison summary.

Note that ttx_diff is a pure Python package (located in src/ttx_diff/) rather than a typical Rust crate within this workspace.

Installation

From PyPI

pip install ttx-diff

From source

git clone https://github.com/googlefonts/fontc.git
cd fontc/ttx_diff
pip install -e .

Requirements

  • Python 3.10 or higher
  • fontc and otl-normalizer binaries (see below)

All Python dependencies (fontmake, fonttools, etc.) are installed automatically.

Getting fontc and otl-normalizer

The tool needs the fontc and otl-normalizer binaries. You can:

  1. Specify paths explicitly (recommended for most users):

    ttx-diff --fontc_path /path/to/fontc --normalizer_path /path/to/otl-normalizer source.glyphs

  2. Add them to your PATH: If fontc and otl-normalizer are in your PATH, they'll be found automatically

  3. Run from fontc repository: If you run from the fontc repository root, the tool will automatically build the binaries for you

Usage

Note: Unlike the original ttx_diff.py script, this standalone version can be run from any directory. You don't need to be in the fontc repository.

Rebuild with both fontmake and fontc and compare:

ttx-diff --fontc_path /path/to/fontc --normalizer_path /path/to/otl-normalizer path/to/source.glyphs

If the binaries are in your PATH:

ttx-diff path/to/source.glyphs

Compare two precompiled fonts directly (without building from source):

ttx-diff --fontc_font path/to/fontc.ttf --fontmake_font path/to/fontmake.ttf

This is useful for comparing font differences when you already have both builds.

Rebuild only fontc's font and reuse existing fontmake output:

ttx-diff --rebuild fontc path/to/source.glyphs

Output results in machine-readable JSON format, as used by the fontc_crater tool.

ttx-diff --json path/to/source.glyphs

Compare using gftools build pipeline:

ttx-diff --compare gftools --config config.yaml path/to/source.glyphs

Development

Running tests

pip install -e .[test]
pytest

Running tests with coverage

pytest --cov=ttx_diff --cov-report=html

Releasing

See https://googlefonts.github.io/python#make-a-release.